Yes, the variable definitions page leaves me scratching my head. Not sure what it's doing under performance. Surely that's a matter of your personal coding style and I'm not sure a jQuery tutorial site should be so dogmatic about how you choose to define your variables when there's negligible performance impact either way. Although JSLint loves to complain about the "old & busted" style, grouping variable definitions together with commas goes against the principle that inserting a new line of code anywhere should be "safe". If you paste in a new variable definition statement with a semicolon in the midst of a group of comma-separated lines, you're going to break things.
Indeed, some of them are basic optimization techniques that experienced jQuery users have 'discovered' over the years (through Google or otherwise) and they are essential to creating high-quality web applications.
Having a central resource is obviously a boon to the entirety of the community and it reduces the barrier to entry, and I personally know some designers (who get by on the bare minimum of jQuery knowledge) that would benefit greatly from this service.
Without the easy DOM manipulation, jQuery's not much more than a nice wrapper to deal with AJAX browser differences. There's some serious cognitive dissonance here.